Problem: In chemistry class, the girl to boy ratio is $4$ to $7$. If there are a total of $33$ students, how many boys are there?
Explanation: A ratio of $4$ girls to $7$ boys means that a set of $11$ students will have $4$ girls and $7$ boys. A class of $33$ students has $3$ sets of $11$ students. Because we know that there are $7$ boys in each set of $11$ students, the class must have $3$ groups of $7$ boys each. There is a total of $21$ boys in chemistry class.